This invention relates to a light quantity setting value determination apparatus and method, and more particularly to a laser printer or other image forming apparatus having a light quantity setting value determination apparatus and method, which writes an image with light by means of a laser beam, a digital duplicator, and other such kinds of image forming apparatus, and even more particularly to a technology in which light emission of a laser diode( light emitting means) is controlled so that a plurality of different predetermined light quantities emitted by the laser diode are set.
A typical example of such an image forming apparatus is described below with reference to FIG. 1. FIG. 1 shows a block diagram of a writing-with-light part of a laser printer of a first embodiment according to the present invention and of the related art.
A laser diode(LD) driver 2 drives a laser diode 1 so as to make it emit light in response to a video signal as a result of modulation. Laser light emitted from the laser diode 1 is deflected by a polygon mirror 3 so that the deflected light is raster-scanned on a photosensitive body 5 via f.theta. lens 4 and a mirror or other light processing parts, resulting in an electrostatic latent image being formed on the photosensitive body 5.
The quantity of the laser light is controlled by an electric current Iop, which flows through the laser diode 1. The electric current Iop is controlled by data, which is set in a digital/analog(D/A) converter 7 by means of a CPU 6.
FIG. 2 shows a relationship between the electric current Iop flowing through the laser diode 1 and a quantity P of light emitted from the laser diode 1 as a result of the electric current Iop flowing through the laser diode 1.
This relationship varies depending on a particular laser diode, variations of temperature, or a period of time which has elapsed since the electric current Iop was generated, as shown, for instance, by curves a, b, and c of FIG. 2. Thus, the above mentioned relationship cannot be predetermined.
Therefore, electric current flowing through the laser diode 1 should be adjusted as required to make the diode 1 emit a desired quantity of light.
Controlling of this light quantity by automatic adjustment is called APC(auto power control ).
APC can be performed by using a photo diode 8 which senses a laser light quantity. A part of the laser light emitted from the laser diode 1 is applied to the photo diode 8, then the photo diode 8 allows an electric current Im to flow, the quantity of which current Im is in proportion to a quantity of the light of the part of laser light applied to the photo diode 8. The current Im is converted into a voltage Vm(a monitoring voltage) by means of a resistance 9, then the voltage Vm is measured by means of an operational amplifier 10 so that the laser light quantity emitted from the laser diode 1 can be measured.
Further, a measuring voltage provided from the amplifier 10 is compared with respective predetermined reference voltages(in this case, 1.2 volts and 2.0 volts) by means of respective comparators 11 and 12. Then, results of the comparison are each supplied as feed back signals to the CPU 6 through respective ports 13 and 14.
Then, the CPU 6 sets certain data in the D/A convertor 7 as a result of its checking the above mentioned feed back signals so as to control the LD driver 2.
A ROM(read only memory) 15 having a preliminary program stored therein to be executed by the CPU 6 and a RAM(random access memory) 16 used as a working memory by the CPU 6 are each connected to the CPU 6.
FIG. 3 shows a relationship between the light quantity P emitted from the laser diode 1 and a monitoring current Im flowing through the photo diode 8 as a result of the light quantity P being sensed. FIG. 4 shows a relationship between the above mentioned light quantity P and the monitoring voltage Vm as a result of the light quantity P being sensed via the photo diode 8 and the resistance 9.
As shown in FIG. 3, the current Im flowing through the photo diode 8 is 0.6 milliamperes(mA) when the laser diode 1 emits 3 milliwatts(mW) of a first light quantity P.sub.1. Then, an electric potential occurs in the resistance 9, that is, the monitoring voltage Vm becomes 1.2 volts(V) if a resistance value of the resistance 9 is 2 kiloohms(k.OMEGA.). Then this 1.2 V of Vm puts the comparator 11 in an inverting condition where the comparator 11 is inverted. That is, APC of P.sub.1 of 3 mW(controlling light quantity emitted by the laser diode 1 to be P.sub.1 of 3 mW) is realized by an operation of the CPU 6 in which a set value Ds to be set in the D/A converter 7 is determined, the comparator 11 being put in the above mentioned inverting condition when the set value Ds is set in the D/A converter 7.
After the set value Ds has been determined once, a subsequent operation can soon be executed using such a value Ds, the subsequent operation being one in which writing-with-light using an adjusted light quantity is performed just at the moment when the subsequent operation is needed without having to wait for the value Ds to be determined.
Then, parts in the writing-with-light-part except for image printing process means such as the above mentioned polygon mirror 3, the f.theta. lens 4, and the photosensitive body 5, act as a light quantity setting value determination apparatus.
There may be a case where the laser diode 1 has to emit another light quantity P.sub.2 instead of, for example, P.sub.1 of 3 mW, soon after the completion of the above mentioned APC of P.sub.1 3 mW. In such a case, APC for at least two light quantities P.sub.1 and P.sub.2 should be executed at the same time.
There are two purposes why the laser diode 1 should emit a plurality of light quantities as mentioned above(in the above example, there are two light quantities). A first purpose is to realize so called shading compensation by which a laser light quantity should be changed to correspond to various positions on a laser scanning line on the photosensitive body 5. Such compensation is needed to compensate for a variation in the darkness of a printed image on a sheet of paper, which variation in darkness may be caused, even if the electric current Iop is constant, due to a certain reason related to light distribution characteristics of an image printing process which employs the laser diode 1, the polygon mirror 3, the f.theta. lens 4 and the photosensitive body 5. A second purpose is to set a desired darkness of the printed image.
The writing-with-light part as shown in FIG. 1 can execute APC for both 3 mW and 5 mW of light quantities. As shown in FIG. 1, in a case where APC for a plurality of light quantities can be executed, a plurality of comparators are provided in the related art for generating a plurality of feed back signals.
An operation of the writing-with-light part a shown in FIG. 1 is described below with reference to FIG. 5(an operational flow chart), and FIGS. 6 through 8(time charts of outputs of the comparators 11 and 12 ).
After APC is started, data(being referred to as "D/A set value Ds" hereinafter, and acting as a light quantity setting value) set in the D/A converter 7 is increased in a step(the word "step" is omitted for the sake of simplifying the description hereinafter) S2 in FIG. 5. Then, when the light quantity emitted by the laser diode 1 becomes equal to a first light quantity P.sub.1, the output of the comparator 11(referred to as "COMP.11" in FIGS. 6B through 8B) is inverted from "0" to "1" at a time t.sub.1 shown in FIG. 6B. Thus, S3 is executed after S1 in FIG. 5, that is, the D/A set value Ds(referred to as "D/A VALUE Ds" in FIG. 5) at the current time is saved(stored).
After this, the D/A set value Ds is further increased in S5, then the light quantity emitted by the laser diode 1 becomes equal to a second light quantity P.sub.2. Then, the output of the comparator 12( referred to as "COMP.12" in FIGS. 6A through 8A) is inverted from "0" to "1" at a time t.sub.2 shown in FIG. 6A. Thus, S6 is executed after S4 in FIG. 5, that is, the D/A set value Ds at the current time is saved(stored ). Then, APC is finished.
However, if an error is included in information fed back from the comparator 11, after APC has started, even if the light quantity emitted from the laser diode 1 exceeds the first light quantity P.sub.1, the output of the comparator 11 cannot be inverted, as shown(as a solid line) at t.sub.1 in FIG. 7B. Thus, the CPU 6 still continues to execute S2 in FIG. 5, that is, the D/A set value Ds is further increased by an undesirable amount resulting in damage occurring in the laser diode 1.
Similarly, if an error is included in information fed back from the comparator 12, after completion of the setting operation of the first light quantity P.sub.1 in the laser diode 1 by means of the CPU 6 via D/A converter 7, even if the light quantity emitted from the laser diode 1 exceeds the second light quantity P.sub.2, the output of the comparator 12 cannot be inverted as shown(as a solid line) at t.sub.2 in FIG. 8A. Thus, the CPU 6 still continues to execute S2 in FIG. 5, that is, the D/A set value Ds is further increased by an undesirable amount resulting in damage occurring in the laser diode 1.
Further, in a case where an abnormal condition occurs in either of the comparators 11 or 12, then a normal APC cannot be executed until this abnormal condition is removed.